Aero propellers, either single rotor or contra-rotating, usually have a means of varying the blade pitch via a pitch control mechanism (PCM). Blade pitch variation can optimise efficiency of thrust delivery whilst reducing noise throughout the flight envelope and provide reverse thrust. Further, by feathering of the blades, drag and rotor speed can be controlled in some failure cases. In particular, relative to a fine pitched propeller blade, a coarse pitched blade generally provides greater rotational resistance (which lowers engine RPMs) and provides less forward velocity drag resistance.
There are a number of established ways of configuring a PCM, but generally they all feature: a source of power, a prime mover, a mechanism from is prime mover to blade, and a failsafe system.
In the event of PCM failure, it may be desirable to move the blades to coarse to prevent dangerous increases in engine speed. In the event of engine failure, it may be likewise be desirable to move the blades to coarse to reduce aircraft gliding resistance. However, the combined effect of rotational and aerodynamic forces acting on the blades tends to urge the blades to fine. Thus PCMs usually have a failsafe arrangement for preventing undesirable pitch variation in the event of power loss failure.
FIG. 1 shows schematically a longitudinal cross-section through a prior art screw pitch lock apparatus for varying the pitch of a row of propeller blades of a propeller assembly. The apparatus comprises a hydraulic cylinder 1 and piston 2 which extend along the rotational axis X of the propeller blades 3 (only one of the propeller blades being shown in FIG. 1). The cylinder contains hydraulic fluid (e.g. oil), and a wall 4 fluidly seals the end of the cylinder. The piston divides the cylinder into two chambers 5, 6. By varying the fluid pressure difference between the two chambers, the piston can be moved to the left or the right along the axis X.
A quill 7 extends radially inwardly from the inboard end of each propeller blade 3 along the rotational axis Y of the blade, the quill connecting to an end of a crank arm 8 which has its other end in a respective retaining recess 9 formed at the end of the piston 2. By this mechanism, movement of the piston along the rotational axis X is converted into pitch-changing rotation of the blade about rotational axis Y.
The cylinder 1 is part of a larger housing which also provides a fixing arrangement 10 for the propeller blades 3 and a rotation drive input 11 for turning the propeller assembly. The drive input is typically connected to the output shaft of an engine gearbox. Hydraulic fluid for the chambers 5, 6 is provided by a fluid transmission tube 12 which extends axially from the drive input. A rotating fluid coupling 13 at the end of the tube allows fluid to be transmitted between the static and rotating fields.
A ball screw 14 (i.e. a screw with a plurality of balls located in the thread of the screw) extends along the rotational axis X, an end of the ball screw 14 being fixed by a hydraulically signalled brake 15 to the wall of the cylinder 1. A nut 16 which is axially and rotationally fixed relative to the piston 2 is threadingly engaged to balls of the ball screw. Lubricated in the hydraulic fluid, the balls provide a low friction threaded connection between the screw and the nut and offer little resistance to the axial movement of the piston in the cylinder whilst the pressurised de-activated brake allows the screw to rotate. However, in the event of fluid pressure loss, the brake activates and increases the frictional resistance to rotational movement of the screw, which restrains movement of the nut and piston and thereby prevents changes to the pitch of the propeller blades 3 in the fine direction.
Screw pitch lock apparatuses, such as the one shown in FIG. 1, require the propeller assembly to have a central zone along its rotational axis for installation of the apparatus. Generally, such a zone is available on single propeller engines where the propeller assembly is mounted to one side of the engine's drive gearbox. However, other engine arrangements, and particularly in-line arrangements, may not have this zone available. For example, EP A 1881176 describes a contra-rotating propeller engine with a pair of propeller blade assemblies which rotate in opposite directions as a result of association with a coaxial epicyclic gear assembly acting as a differential gearbox. The propeller assemblies are in the “pusher” configuration, with the free power turbine drive shaft, static support structure for the propeller assembly rotors and the gearbox occupying central space on the axis of the forward propeller assembly, and thereby rendering a centrally-located ball screw style pitch lock apparatus impractical for at least the forward propeller assembly.
Likewise, a centrally-located ball screw style pitch lock system would be impractical for the rear propeller assembly of a propeller engine with a pair of contra-rotating “puller” propeller blade assemblies driven by an in-line gear assembly.